Ithaca Knick Single Barrel Trap Gun Problem

After disassembly of my Ithaca Single Barrel Trap Gun, into the three major parts, I inadvertently pulled the trigger. How do I recock the gun, to enable me to put the gun back together? I have tried to replace the barrel, thinking that would work but, to no avail. The opening lever returns to center, but does not cock the hammer/trigger. Does anyone own this type of gun and know how to correct my problem?
Also, what is the year of manufacture with serial number 404726?
Lee Texter

Lee, the reason the hammer is not staying cocked is because the sear is not engaging the hammer. If you removed the sear pin and sear make sure you put the sear back the right way. With the trigger plate off and the barrel along with the forend installed you should see the hammer coming back and engaging the sear to lock back when you open the gun.